1. Field of the Invention
The invention relates to a crosslinkable powder composition which is redispersible in water and also to its use.
2. Background Art
EP-A 601518 describes crosslinkable dispersion powders which are redispersible in water and based on carboxyl-containing acrylate copolymers which are dried in the presence of polyvinyl alcohol and can contain polyvalent metal ions for crosslinking. Similar crosslinkable dispersion powders are also known from U.S. Pat. No. 3,409,578 in which powder compositions comprising carboxyl-containing polymers and/or carboxyl-containing protective colloids are crosslinked in the presence of polyvalent metal ions. A disadvantage is that such compositions comprising polyvalent metal salts which liberate acid in the presence of water tend to crosslink prematurely, especially in contact with moisture.
To avoid premature crossslinking, EP-A 702059 (U.S. Pat. No. 5,608,011) proposes using crosslinkable dispersion powders comprising N-methylol-containing copolymers and a catalyst combination of water-soluble or water-dispersible peroxo acid salts or reducing agents. EP-B 687317 describes crosslinkable powders which are redispersible in water and are based on N-methylol-functional copolymers which crosslink on heating. A disadvantage of the latter powder is the fact that exclusively heat-crosslinkable powders cannot be used in many applications.
EP-A 723975 discloses crosslinkable dispersion powders based on copolymers containing epoxide groups. For crosslinking, external crosslinkers such as polyamines, polycarboxylic acids, hydroxy-functionalized polymers and polyvalent metal salts are added. A problem with the use of polyamines or polyvalent metal salts is the low storage stability. The use of polycarboxylic acids or hydroxy-functionalized polymers does not impair the storage stability; however, elevated temperatures have to be employed for the reaction with the epoxy groups since otherwise the stabilization indicated in the application of the dispersion powders using customary protective colloids would not be possible.
EP-A 721004 discloses crosslinkable, redispersible powder mixtures comprising film-forming polymer particles having at least one functional, crosslinkable group. In this document too, it is recommended that crosslinking be carried out by addition of an external crosslinker which is soluble or dispersible in water and comprises at least one reactive component which forms nonionic bonds with the functional groups after the mixture is redispersed in water. The crosslinker component can be a water-soluble or emulsifiable monomer or polymer, or else a pulverulent mixture of the crosslinker and an inorganic carrier such as CaCO.sub.3, clay or silicon dioxide. The crosslinker component is bound to these carrier substances by physical adsorption. This has the disadvantage that these mixtures have the same odor, if somewhat attenuated, as the pure crosslinker component. In addition, owing to the weak physical bonding, the crosslinker component can become detached and penetrate into the polymer to be crosslinked, which, particularly in the case of very reactive substances, can lead to a reduced shelf life or premature crosslinking. This applies particularly when the protective colloid component has functional groups.
It is an object of the invention to provide a crosslinkable, film-forming polymer powder which is dispersible in water and has good storage stability, blocking resistance and is free-flowing.